1. Field of the Invention
This invention relates to acoustooptic devices, and more particularly to such devices for intensity modulating and diffracting a light beam.
2. Description of the Prior Art
Diffraction of light by high-frequency acoustic waves is well known. When a light beam 2 of optical wavelength .lambda. forms an angle .alpha. with acoustic wavefronts 4, as shown in FIG. 1, such that EQU sin .alpha.=.lambda./2.LAMBDA., (1)
where .LAMBDA. is the acoustic wavelength, a first order diffracted beam 6 appears to be reflected from the acoustic wavefronts as if they were mirrors. This phenomenon is called acoustic Bragg reflection and the angle .alpha. which satisfies the expression is called the Bragg angle. The diagonal lines in FIG. 1 represent optical wavefronts of incident beam 2 and diffracted beam 6.
The light modulator was the first device to make use of the diffraction of light by ultrasound. Referring to FIG. 2, a well-collimated incident beam 10 of light passes through a diffraction cell 12. An acoustic beam of ultrasound, traveling through the cell across the light beam, diffracts some of the incident light into a diffracted beam 14 to generate an optical output, the intensity of which is a function of the acoustic power. Some incident light passes through the cell as an undiffracted beam 18.
When the acoustic power is changed by amplitude modulation of the electrical information signal applied to an electro/mechanical transducer 16 in order to vary the intensity of the diffracted light beam, the acoustic wavefronts corresponding to the new signal travel across the light beam at sound velocity. Until acoustic wavefronts corresponding to the new signal have completely replaced those corresponding to the old signal in the light beam, a portion of the diffracted beam will be at one intensity while another portion will be at another intensity. Therefore, if one desires to modulate the light intensity at a fairly fast rate of intensity change, the incident light beam must be quite narrow in the direction of acoustic wave propagation. In other words, the speed that the intensity of the diffracted beam can be changed across the light beam is a function of the time that it takes for an acoustic wave to cross the beam. This time, commonly referred to as the access time .tau. (and referred to herein as .tau..sub.m to denote the modulation access time), is determined by the acoustic velocity v and the width of the light beam D as follows EQU .tau..sub.m =D/v. (2)
Accordingly, for quick response, one would want the beam width D to be small.
While in the preceding description, it was pointed out that the intensity of the deflected and undeflected beams are modulated by the power applied to transducer 16, it is also true that the light beam is deflected by ultrasound based on a linear relationship between the acoustic frequency and the sine of the Bragg angle. Substituting the quotient of sound velocity v and sound frequency f for the sound wavelength .LAMBDA. in equation (1),: EQU sin .alpha.=f.lambda./2v. (3)
Since Bragg angles are usually small, sin .alpha. can be approximated in equation (3) by .alpha. so that 2.alpha. represents the angle by which the diffracted beam departs from the path of the incident beam as shown in FIG. 2. To vary the direction of the diffracted beam, one varies f such that: EQU .DELTA.(2.alpha.)=.DELTA.f.lambda./v. (4)
In imaging systems, what is frequently most critical is not the magnitude of deflection angles obtainable (since the magnitude of the angles can be magnified by lenses), but rather the number of angular positions that can be clearly distinguished from each other, usually called the number of resolvable spots and denoted by the symbol N. The smallest resolvable angle .alpha..sub.min is conventionally approximated by .lambda./D. To determine the number of resolvable spots, one divides the maximum angular displacement .DELTA.(2.alpha.) by the smallest resolvable angle of a light beam projected from an aperture D (FIG. 2), or: ##EQU1## where .tau..sub.d represents the deflection access time, the aperture is uniformly illuminated and the scan time is in excess of the access time.
Thus, we see that in systems employing both modulation and deflection, the access time .tau..sub.m must be short (small aperture D) for quick response, while the access time .tau..sub.d must be long (large aperture D) for a suitable number of resolvable spots. In such systems, as shown in FIG. 3, it is common practice to use beam contracting and expanding optics for the modulator and to use sheet-beam-forming optics for the deflector. For example, in FIG. 3, the relatively large, pencil-like beam 20 from a laser 22 is concentrated by a spherical lens 24 and recollimated as a narrow beam 25 by another spherical lens 26. Narrow beam 25 passes through the acoustic beam of an acoustooptic modulator 27. Beyond modulator 27 a lens arrangement 28 includes two spherical lenses which expand the modulated beam to its original diameter. A cylindrical lens 30 shapes the beam to form a long, narrow waist centrally within a diffraction cell 31. The longest cross-sectional beam axis within cell 31 is aligned with the direction of sound propagation within the cell. Lens 32 serves to reshape the light into its original, circular cross section form. Consequently, within the modulator, a narrow, pencil-like beam interacts with the acoustic beam over a minimum thickness to minimize response time, and, within the deflector the wide, flat optical beam interacts with the acoustic beam over a maximum thickness for maximum resolution.
While the illustrated prior art arrangement results in fast modulator access and good deflector resolution, the optics required are expensive, cause an undue amount of light loss and extend the overall system length required. By the present invention, the need for optics between the modulator and the deflector has been eliminated by directing light-modulating acoustic beams across the sheet beam formed for the deflection operation. Those acoustic beams, unlike the light-deflecting acoustic beams, propagate along the shortest axis of the sheet beam.